A so-called Czochralski method has heretofore widely been adopted in production of single crystal material such as single crystal semiconductor material. This method comprises steps of melting polycrystal silicon in a vessel into a melt, dipping an end portion of a seed crystal into the melt and pulling the seed crystal while rotating, wherein a single crystal with the same crystal orientation as that of the seed is grown on the seed. A quartz glass crucible is generally used as the vessel for pulling silicon single crystal.
In a general case of pulling a silicon single crystal, a quartz glass crucible holds a silicon melt at a temperature higher than 1400.degree. C. for a long time. While this holding time changes depending on various pulling conditions, it usually takes tens of hours or, in the longest case, more than 100 hours to complete pulling of a silicon single crystal(s) from a melt in a quartz glass crucible with an outer diameter of 20 inches or more.
In such a long-time pulling case, cristobalite in brown color is produced in small ring like patterns on a surface of a quartz glass crucible whose surface has been put in contact with a silicon melt and as time elapses, the patterns change in shape and grow in size on the surface of a quartz glass crucible (see FIG. 3). It has been known that when a portion encircled by this brown ring is eroded to expose a rough surface, dislocations are apt to be generated in a silicon single crystal in growth, which causes inconvenience in the pulling.
As shown in FIG. 3, the brown ring does not necessarily keep its original ring shape in the course of growing larger in size, but for simplicity of description, modifications or alterations evolved from the brown rings are generally referred to as brown ring throughout the present specification hereinafter.
A brown ring described above, as shown in FIGS. 3(i) to 3(iv), is created as a ring pattern (B) in brown color (see FIG. 3(ii)), on an inner surface (A) (see FIG. 3(i)) of a normal quartz glass crucible at the outset, while the shape of the brown ring changes and broadens in various way, as time elapses, and a white cristobalite layer (C) is then formed in a portion encircled by the brown ring (B). As time further elapses, devitrified specks (D) come out in the white crystobalite layer in an irregular manner (see FIG. 3(iii)) and as time still further elapses, the portion encircled by the brown ring (B) is gradually eroded to start exposing a glass dissolving surface (amorphous) (E) with a rough appearance (see FIG. 3(iv)). Once this glass dissolving surface appears, dislocations are apt to be generated in a silicon single crystal in growth, which entails reduction in crystallization ratio.
There has been a proposal of a crucible whose inner wall is formed with crystobalite as disclosed in Japanese Patent Laid-open Publication No.9-52791. The proposed method, however, is such that synthetic quartz is molten and then gradually cooled to obtain blocks of crystobalite quartz, the blocks are cut into pieces each in a proper shape and the pieces are welded on a quartz glass crucible. Hence, the method is not satisfactorily accepted in terms of productivity of crucibles.
As shown in Japanese Patent Laid-open Publication No.8-2932, there has been another proposal of a method of preventing roughening of a crucible inner surface during pulling a silicon single crystal by formation of a crystalline layer on the crucible inner surface, wherein the crystalline layer is produced by forming a coating film or a solid solution layer which contains a crystallization accelerating agent in thickness of 1 mm or less from the inner surface portion of a quartz glass crucible for pulling a silicon single crystal. According to this method, however, impurities are mixed into a single crystal through a silicon melt or an atmosphere surrounding the melt when adding the crystallization accelerating agent, the impurities disadvantageously serving as causes of increase in crystal defects of the single crystal.
With the foregoing problems of the prior art in view, an object of the present invention to provide a quartz glass crucible for pulling a silicon single crystal and a production process for the crucible, wherein an inner surface of the crucible is crystallized without addition of impurities during pulling a silicon single crystal, thereby impurities serving as causes of crystal defects being not incorporated into the silicon single crystal, so that deterioration of its inner surface is suppressed to improve a crystallization ratio, and accordingly productivity of the quartz glass crucible as well as a quality of the silicon single crystal is improved.